This is an application for a Mentored Clinical Scientist Development Award (K08) from NINDS. The applicant is completing training as an Epilepsy specialist, and is interested in understanding the behavior of networks of neurons in normal and pathological conditions. The applicant has previous experience using computer models and wishes to learn the complementary experimental techniques in order to pursue this interest with a combination of theoretical and experimental approaches. The long-term goal is to achieve insight into the pathophysiology of epilepsy in order to be able to better treat patients. The goal of this award is to acquire the skills and experimental techniques necessary to become an independent investigator. The goal of the proposed studies is to explore the role of pregnenolone sulfate (PS) in the modulation of inhibitory synaptic transmission. PS, one of the more common steroids in the central nervous system (CNS) inhibits GABA receptor-mediated currents. PS is present in the CNS at high nanomolar concentrations, but the effective concentration had been thought to be in the micromotar range suggesting that PS does not cause physiologically relevant modulation. However, our preliminary data suggest that PS inhibition of GABA receptor-mediated currents is activation-dependent, requiring activation of receptors in order for modulation to occur. Activation dependence may allow modulation at PS concentrations lower than previously appreciated. The objective of this proposal is to further characterize the properties of PS inhibition of GABA receptors. Experiments designed to explore the relative importance of ligand binding and channel opening for optimal PS effect, define the kinetic properties of a PS insensitive mutant GABA receptor and compare the properties of PS inhibition to those of zinc, an important endogenous GABA modulator, will be performed using Xenopus oocytes expressing GABA receptors and native receptors in hippocampal microisland cultures and acute hippocampus slices. Computational modeling will be employed to provide a theoretical foundation to facilitate interpretation of the data. Appreciation of the properties of PS inhibition may result in a better understanding of the conditions under which PS may be an important endogenous modulator of GABA currents and provide insight into the underlying properties of GABA receptors.